Liquid crystal display

ABSTRACT

A liquid crystal display including: a first transparent substrate coated with a first alignment layer, a second transparent substrate coated with a second alignment layer, the second substrate facing the first transparent substrate, a liquid crystal layer between the substrates, a polarizer attached on the outer surfaces of the substrates, a pair of electrodes formed on the first substrates, and a driving circuit applying signal voltage to the electrodes. The liquid crystal molecules adjacent to the first substrate is rotated by applying the voltage, but, the liquid crystal molecule adjacent to the second substrate is fixed regardless of the applied voltage. The electrode pair, substantially straight data and common electrodes, are inclined at an angle with respect to a gate line.

This application is a divisional of prior application Ser. No.09/982,830, filed Oct. 22, 2001 now U.S. Pat. No. 6,781,660; which is adivisional of prior application Ser. No. 09/982,836, also filed Oct. 22,2001 now U.S. Pat. No. 6,903,792; which is a divisional of priorapplication Ser. No. 09/768,241, filed Jan. 25, 2001 now U.S. Pat. No.6,317,183; which is a continuation of application Ser. No. 09/365,634,filed Aug. 3, 1999 now U.S. Pat. No. 6,323,927; which is a continuationof application Ser. No. 08/832,980, filed Apr. 4, 1997 now U.S. Pat. No.5,995,786.

This application claims the benefit of Korean Application No. 10152/1996filed on Apr. 4, 1996, which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display, and moreparticularly to an IPS (In-Plane Switching) liquid crystal display thatis large in area and exhibits a wide viewing angle.

Conventional TFT LDCs (thin film transistor liquid crystal display) havea drawback known as a viewing angle dependency, that is, the contrastratio is changed according to the viewing angle. This has made itdifficult to apply the technology to a large size display.

To solve this problem, various liquid crystal displays have beenproposed, such as a retardation attaching TNLCD (twisted nematic liquidcrystal display) and a multi-domain liquid crystal display. The LCDsstill have other technical problems such as complicated productionprocess and shifting color tones.

2. Discussion of Related Art

Recently, an IPS LCD has been introduced to obtain a wide viewing angle.This technology is discussed in JAPAN DISPLAY 92, p547, Japanese patentapplication No. 7-36058, Japanese patent application No. 7-225538, andASIA DISPLAY 95, p707. As shown in FIG. 1 a and FIG. 1 c, in the liquidcrystal layer 12 the molecules are aligned at a 45° angle. The principaltransmittance axis of a polarizer 9 attached to the first substrate 1 isthe same direction as the alignment direction of the liquid crystal 12,and the principal transmittance direction of an analyzer 10 attached tothe second substrate 5 is perpendicular to the alignment direction ofthe liquid crystal layer 12. A pair of electrodes 2, 3 is formed on thefirst substrate 1.

In FIG. 1 b and FIG. 1 d, when the voltage is applied between twoelectrodes, a horizontal electric field is created. Therefore, thetransmittance is controlled by causing the liquid crystal molecules tobe rotated to be parallel with the electric field. When the rotationangle of the liquid crystal molecules is 45° in the normally black mode,the retardation value (Δnd) is about λ/2(0.21–0.36 μm) for a maximumtransmittance.

In conventional IPS LCDs as described above, the transmittance iscontrolled by birefringence, and a retardation film is necessary tocompensate for the viewing angle which increases the manufacturing cost.

In addition, a viewing angle inverted area appears in the centralportion of the outer lines of the display.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a liquid crystaldisplay that substantially obviates one or more of the problems due tolimitations and disadvantages of the related art.

An object of the present invention is an IPS mode liquid crystal displayhaving a wide viewing angle and improved picture quality.

Another object of the present invention is an IPS mode liquid crystaldisplay that can be fabricated at low cost by using a low voltagedriving IC and by eliminating the need for a retardation film.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, the liquidcrystal display device of the present invention includes a firstsubstrate having a first surface and a second surface, a first alignmentlayer formed on the second surface of the first substrate, a secondsubstrate having a first surface and a second surface, a secondalignment layer formed on the second surface of the second substrate, amolecular liquid crystal layer between the second surface of the firstsubstrate and the second surface of the second substrate, a pair ofelectrodes formed in parallel on the second surface of the firstsubstrate, a polarizer formed on the first surface of the firstsubstrate and having a transmittance axis, and an analyzer formed on thesecond surface of the second substrate and having a transmittance axis.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIGS. 1 a, 1 b, 1 c, and 1 d schematically show cross-section and planviews of a pixel of a conventional liquid crystal display;

FIGS. 2 a, 2 b, 2 c, and 2 d schematically show cross-section and planviews of a pixel of a liquid crystal display according to the presentinvention;

FIG. 3 shows the optical axes of the liquid crystal display according tothe present invention;

FIG. 4 a shows a plan view of the liquid crystal display according tothe present invention, and FIG. 4 b shows a cross-section of the devicetaken along the line IV—IV of FIG. 4 a;

FIG. 5 shows a pixel electrode pattern according to the presentinvention;

FIG. 6 shows a cross-sectional view taken along the line V—V of FIG. 5;

FIG. 7 shows a driving wave pattern of a TFT LCD according to thepresent invention; and

FIG. 8 shows the relationship between the wave length of data voltageand the transmittance of a TFT LCD according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

The liquid crystal display device of the present invention, as describedbelow, comprises a first transparent substrate, a second transparentsubstrate facing the first transparent substrate, an alignment layercoated on the substrates, a liquid crystal layer injected between thesubstrates, a polarizer attached on the outer surfaces of thesubstrates; a pair of electrodes formed on the first substrates, and adriving circuit applying a signal voltage to the electrodes. Liquidcrystal molecules adjacent to the first substrate are rotated byapplying the voltage, however, liquid crystal molecules adjacent to thesecond substrate are fixed regardless of the applied voltage.

An embodiment of the display according to the present invention is shownin FIGS. 2 a–2 d. FIG. 2 a and FIG. 2 c are a cross-sectional view and aplan view, respectively, when a driving voltage is not applied to thedisplay and FIGS. 2 b and 2 d are a cross-sectional view and a planview, respectively, when the driving voltage is applied.

When the driving voltage is not applied, the liquid crystal moleculesadjacent to the first substrate 27 are aligned with the liquid crystalmolecules near the second substrate 26 as shown in FIGS. 2 a and 2 c.The liquid crystal molecules adjacent to the substrates are aligned bythe alignment directions induced on alignment layers 59, 62.

The liquid crystal molecules 78 will be twisted between the twosubstrates 26, 27 by applying the driving voltage to the electrodes 48,49. The distance between two adjacent electrodes 48, 49 is less than thethickness of the liquid crystal layer 60. The retardation value iscalculated by following formula:

Wherein, Δn is the dielectric anisotrophy, d is the thickness of theliquid crystal layer, and λ is referred as a wave length.

In addition, when the horizontal line of the liquid crystal display is0°, the angle of the electric field (θ_(FE)) applied by the electrodesis according to the following formula:0°<θ_(FE)<90°,0°<θ_(FE)<−90°

FIG. 3 shows optical axes of the liquid crystal display according to thepresent invention. θ_(EL) is represented as the extension direction ofthe electrodes 48 and 59, θ_(FE) is the electric field direction appliedby the electrodes, θ_(LC1) is the optical axis direction of the liquidcrystal molecules adjacent to the first substrate and θ_(LC2) is theoptical axis direction of the liquid crystal molecules adjacent to thesecond substrate when the voltage is not applied. θ_(PL1) is theprincipal transmission axis of a polarizer, θ_(PL2) is the principaltransmission axis of an analyzer, θ_(LC1) is the optical axis directionof the liquid crystal molecules adjacent to the first substrate when thevoltage is applied. The alignment direction θ_(LC1) of the firstsubstrate is anti-parallel to the alignment direction θ_(LC2) of thesecond substrate, and the principal transmission axis θ_(PL2) of theanalyzer is parallel to the alignment direction θ_(LC1), θ_(LC2). Inaddition, the principle transmission axis θ_(PL1) of the polarizer isperpendicular to the principal transmission axis θ_(PL2) of theanalyzer.

The extension direction of the electrodes, θ_(EL), is slightly slantedcompared to the conventional extension direction which is 90° relativeto the horizontal line 0° of the substrate as shown in FIG. 3.

By forming the electrodes on the slant, when the voltage is not applied,all of the liquid crystal molecules between the two alignment layers 59,62 are aligned parallel to the substrates 26, 27 and to the principaltransmission axis θ_(PL2) of analyzer. Therefore, the viewing angleinverted areas appear at the corners of the display so that the invertedphenomenon is not remarkable. The liquid crystal is nematic without theneed to mix a chiral dopant. The LCD shown in FIG. 2( a) is a normallyblack mode because the polarizer and the analyzer are crossed and theliquid crystal molecules between them are parallel to one another.

One of the two electrodes 48 and 49 is called a data electrode and theother electrode is called a common electrode. The parallel electricfield 13 is formed in the θ_(FE) direction by a signal voltage betweenthe data electrode, e.g., electrode 48, and the common electrode, e.g.,the electrode 49. The parallel electric field 13 has a maximum strengthadjacent to the first substrate 27, and a minimum strength adjacent tothe second substrate 26. In the middle of the liquid crystal layer 60,the parallel electric field 13 has a mean strength defined by(E_(M)=(E₁+E₂)/2). The electric field is weaker as the distanceincreases from the first substrate formed with electrodes 48 and 49. Theirregularities in the electric field can be achieved by making thethickness of the liquid crystal layer greater than the interval betweentwo electrodes.

The liquid crystal molecules 77 adjacent to the first substrate 27 arerotated to the electric field direction θ_(FE) by the maximum electricfield. The rotation angle θ_(RT1) is determined by following formula:θ_(RT1)=74 _(LC1)−θ_(LC1)′ and the maximum rotation angle isθ_(LC1)−θ_(FE).

The liquid crystal molecules 78 adjacent to the second substrate 26 aresubject to an electric field that is below the threshold electric fieldso that the molecules retain the original direction θ_(LC2). In thismanner, the liquid crystal molecules are continuously twisted betweenthe two substrates 26, 27. Polarized light having a polarized directionθ_(PL1) of polarizer 63 is guided by the twisted liquid crystalmolecules 60 to the perpendicular direction parallel with the principletransmittance axis θ_(PL2) of the analyzer 64. As a result, a whitestate is obtained by transmitting polarized light through the polarizer63 and the analyzer 64.

The transmittance is dependent on the twisted angleθ_(TW)=θ_(LC2)−θ_(LC1)′, that is, the transmittance is increased inproportion to the degree of the twisted angle. A grey level of theliquid crystal display is controlled by the signal voltage inducing theliquid crystal molecules to be twisted.

To improve the viewing angle, the transmittance axis of the polarizer 63is perpendicular to the transmittance axis of the analyzer 64(θ_(PL1)=0°, θ_(PL2)=90°), and the alignment direction θ_(LC1) of theliquid crystal molecule 77 adjacent to the first substrate 27 isanti-parallel with the alignment direction θ_(LC2) of the liquid crystalmolecule 78 adjacent to the second substrate 26 (θ_(LC1)=90°,θ_(LC2)=−90°) In addition, the extension direction θ_(EL) is angularlyoffset (for example, θ_(EL)=95°) from the electric field directionθ_(FE), which is in a direction perpendicular thereto (for exampleθ_(FE)=5°) such that extensive direction θ_(EL) is slanted. The liquidcrystal molecules 77 adjacent to the first substrate 27 are rotated from90° to 5°, while the liquid crystal molecules 78 adjacent to the secondsubstrate 26 are fixed, so that the rotation angle between the firstsubstrate 27 and the second substrate 26 is 85°.

The retardation value Δnd, which provides a maximum transmittance to theliquid crystal layer 60, is calculated according to the following

[Δnd=0.74λ]Δnd=(85°/90°)λ=0.94. The dielectric anisotrophy Δn and thethickness d of the liquid crystal are appropriately arranged. Thedielectric anisotrophy of the liquid crystal generally used in TN modeis 0.06–0.08, and the wave length of the light is 0.56 μm. When thevalues are substituted in the above formula, the thickness d should be6.0–8.8 μm.

FIG. 4 a shows a plan view of a liquid crystal display according to thepresent invention, and FIG. 4 b shows a cross-sectional view taken alongthe line IVA—IVA of FIG. 4 a. The area outside of the viewing area 21 isprotected by a metal frame 22, an area deposited with a driving circuit23 for a gate line, a driving circuit 24 for a data line, and a backlight housing 25 including a back light 31. The viewing area orpresentation unit 21 is shown in FIG. 4 b to successively comprise aphotoguide plate 75 including a photo-diffusion plate, a polarizer 63, afirst substrate 27, a second substrate 26, and an analyzer 64. Tocompensate for the contrast ratio of the liquid crystal display, aretardation film can be deposited either between the polarizer 63 andthe first substrate 27 or between the second substrate 26 and theanalyzer 64.

This invention can be adopted for use with a diode mode LCD using diodesinstead of TFT, or a simple matrix LCD using a simple matrix substrate,as well as the TFT mode LCD which includes the first substrate 27 formedwith a thin film transistor and the second substrate 26 formed with acolor filter. In addition, this invention can be adopted to monochrometype LCD or TFT mode LCD, which includes the first substrate 27 formedwith a color filter and the second substrate 26 formed with a thin filmtransistor.

FIG. 5 is a plan view showing an electrode pattern for a pixel of thefirst substrate 27. FIG. 6 is a cross-sectional view of a liquid crystaldisplay taken a line V—V of FIG. 5.

The liquid crystal panel is composed of two substrates 26, 27, a liquidcrystal layer 60 and a spacer 65 supporting the thickness of the liquidcrystal layer. The substrates 26, 27 are coated with alignment layers62, 59 on their respective inner surfaces and attached with polarizers64, 63 on their respective outer surfaces. TFT 55 is formed between thefirst substrate 27 and alignment layer 59, and a color filter 61 isformed between the second substrate 26 and the alignment layer 62.

TFT 55 is formed in the intersection of a gate line 41 and a data line42, the gate line 41 extends horizontally (for example 0°) and the dataline extends vertically (for example 90°). A common line 43 passesthrough the center of the pixel in a direction parallel to the gate line41. A common electrode 49 extends from the common line 43 to theinterior inside of the pixel area in a slanted direction relative to thedata line 42. The data electrode 48 is provided parallel to commonelectrode 49 in the pixel area and is connected to a drain electrode 47of TFT 55.

A AlTa thin layer (for example Ta content about 3%) with a thickness ofabout 0.3 μm thick is photo etched to pattern the gate line 41, commonline 43 and common electrode 49. Then, an AlTa oxidation layer 52 isformed to a thickness of about 0.1 μm by anodizing the surface of theAlTa thin layer. Both a gate insulation thick film layer 57 of about a0.3 μm of SiNx and an amorphous silicon (a-Si) layer 44 are patterned bya plasma chemical vapor deposition method. A Cr thin layer about 0.1 μmthick is deposited by a sputtering method and photoetched to form thedata line 42, a source electrode 46, and a drain electrode 47 of the TFT55, and the data electrode 48. The TFT 55 is completed by removing theN+ silicon layer within the channel of the TFT 55. The intersection ofthe common line 43 and data electrode 48 forms a storage capacitor 53,to support an electric charge (voltage) for each pixel. Finally, a SiNxpassivation layer 58 (0.2 μm thickness) is deposited on the entiresurface.

A black matrix 51 and a color filter 61 are formed on the secondsubstrate 26. It is also possible to deposit an overcoat layer on theblack matrix 51 and the color filter 61 to provide stability andflatness for the surface. The black matrix 51 is formed with a thinlayer of width less than 10 μm, for example, 0.1 μm thick Cr/CrOx, onthe area of the gate line 41, the data line 42, and common line 43 toprevent leakage of light therefrom. The color filter 61 is repeatedlyformed with R,G,B layers in each pixel area.

In the above structure, the extension line θ_(EL) is disposed 95°relative to the horizontal line (0°), such that the electric fileddirection θ_(FE) is 5°. The extension line is extended from the 5 μmwidth data electrode 48 and the 5 μm width common electrode 49, whichare parallel to each other with a 5 μm space therebetween.

The alignment layers 59, 62 coated on the first and second substrates27, 26 are obtained by coating, for example, RN1024 (produced in NISSANCHEMICAL CO.) to a thickness of about 8 μm and baking. The alignmentlayer 59 coated on the first substrate 27 is rubbed in the −90°direction, and the alignment layer 62 coated on the second substrate 26is rubbed in the 90° direction. The spacer 65 can be formed fromMicropal (produced in SEKISUI FINE CHEMICAL CO.) with an exemplary 8.0μm diameter, to maintain the liquid crystal layer 60 with a meanthickness of 7.8 μm. The liquid crystal material can be ZGS 5025(Δn=0.067; Δε=6.0; produced by CHISSO CO.). The pretilt angle of theliquid crystal is 4.8°, and the retardation value Δnd is 0.41.

The principal transmittance axis of the polarizer 63 attached on thefirst substrate 27 is the horizontal direction (θ_(PL1)=0°) and that ofthe analyzer 64 attached on the second substrate 26 is the verticaldirection (θ_(PL2)=90°).

The interval (horizontal spacing) between the data electrode 48 andcommon electrode 49 is less than the thickness of liquid crystal layer60. The retardation value of the liquid crystal layer is satisfied withthe following formula:

λ/2<Δnd≦λ, wherein, Δn is a dielectric anisotrophy of the liquidcrystal, d is the thickness of the liquid crystal, and λ is the wavelength of the light.

The electric field direction is satisfied with the following formula:0°<θ_(FE)<90°.

The electro optical characteristics of the above mentioned described TFTLCD are evident with reference to FIG. 7 and FIG. 8.

FIG. 7 shows the driving voltage pulse of the LCD fabricated accordingto the present invention, wherein, the LCD has a 12.1 inch screen, a480×640 (R.G.B) array of pixels. The gate voltage V_(G) 71 isV_(GH)=20V, V_(GL)=0V, the width of the pulse=31 μs, and the commonvoltage V_(CO) 72 is 8V direct voltage. In addition, the data voltageV_(D) 73 is a monowave signal with a pulse width of 31μs, of which themaximum voltage is 6V, the minimum voltage is 1V, and 5V is controlledin the signal area.

FIG. 8 is a graph showing the relationship between the amplitude of thedata voltage V_(D) and the transmittance of the liquid crystal panel.The transmittance has a maximum value of 3.6% near the 5V to datavoltage amplitude. The dotted line shows the transmittance of the LCD inwhich the interval (spacing) between two electrodes is 10 μm and thethickness of the LC layer is 6.5 μm (the interval > the thickness). Thetransmittance is increasing at 6V, and is ¼ of that illustrated by asolid line corresponding to the LCD of the present invention asdescribed herein. Therefore, the LCD according to the present inventionhas a high transmittance despite the use of a low driving voltage.

The rotation angle of the liquid crystal layer is detected by anevaluator for LCD (produced in NIHON DENSHI CO.). The results show thatthe liquid crystal molecules 1 μm distant from the second substrate 26have a optic axis of 88°, and the liquid crystal molecules 1 μm distantfrom to the first substrate 27 have a optic axis of 19°. It can beunderstood that the alignment direction of the liquid crystal moleculesnear the first substrate 26 is almost fixed, but the alignment directionof the liquid crystal molecules near the first substrate 27 is rotatedabout the anticipated angle. At this position, the anticipated angle ofthe liquid crystal molecules is 16. Therefore, the liquid crystalmolecule is twisted in the liquid crystal layer.

The driving voltage is in the range of 1.8V–5.0V, the viewing angle isover ±70° in the vertical direction and over ±70° in the horizontaldirection, wherein the contrast ratio is over 5:1, In this range, thegray level is not inverted. Accordingly, this LCD is remarkably improvedin both vertical and horizontal viewing, can be used with a standard 5Vdriving IC, and has a front contrast ratio of 120:1.

The embodiments of the present invention have been described as having a95° electrode extension line as an example. However, the direction canbe selected according to the viewing angle characteristics that arerequired.

In addition, the alignment layers for the substrates need not be thesame. For example, the alignment layer for the first substrate 27 can bematerials that have a low anchoring energy to allow easy rotation of theadjacent liquid crystal molecules according to the applied drivingvoltage. The alignment layer for the second substrate 26 can be apolyamic-based material having a high anchoring energy liquid crystalmolecules to inhibit rotation under the influence on applied electricfield.

The present invention provides an LCD in which the data line and thecommon line are deposited in a slanted orientation so that the viewingangle is improved. This provides a large rotation angle for the liquidcrystal molecules to permit the thickness of the liquid crystal layer tobe increased, so that a standard (low voltage) driving IC can be used.In addition, a twisted nematic structure can be easily used in an IPSmode LCD without using a retardation film. Reliability is improved byusing conventional twisted nematic liquid crystal.

It is to be understood that the form of the preferred embodiments of thepresent invention shown and described herein are to be taken as apreferred examples of the same and that various modifications andapplications may be resorted to without departing from the spirit of thepresent invention or the scope of the appended claims.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the liquid crystal displayof the present invention without departing from the spirit or scope ofthe invention. Thus, it is intended that the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

1. A liquid crystal display device comprising: a first substrate havinga first surface; a second substrate having a second surface; a liquidcrystal layer formed adjacent to the first surface and the secondsurface; at least one pair of electrodes formed adjacent to said firstsurface in a slanted direction with respect to the first substrate;wherein a distance between the pair of electrodes is greater than athickness of the liquid crystal layer.
 2. The liquid crystal displaydevice according to claim 1, wherein the thickness of the liquid crystallayer is in a range of 5.0–7.0 μm.
 3. The liquid crystal display deviceaccording to claim 1, wherein the slanted direction is an greater than0° and less than 90°.